It's a good question. Judgements that cause direct damage (Command, Righteousness, Vengeance), rather than a debuff, do not remove an existing Judgement. As long as you continue to hit the mob, Crusader will remain up unless you explicitly use a different debuffing Judgement (Wisdom, Light, Justice). Note that the stacking debuff from Seal of Vengeance is not a Judgement and doesn't count against your limit; neither is the stun proc from Seal of Justice.
While your Paladin is leveling up, you won't have access to Seal of Vengeance. So in that circumstance, Seal/Judgement of Righteousness will be your core threat generation abilities. Since you lack a pulling ability until at least level 50 (Avenger's Shield), you will want to consider starting with JoR rather than JotC to frontload threat while tanking.
While soloing, I wouldn't even bother with Crusader, just judge Wisdom to help keep your mana up if you find yourself drinking often. There's really no point in using a full Seal/Judgement rotation on non-elites, as they die too fast to justify the mana.
My favorite Protection trick is to ride through a batch of mobs, aggro all of them, then turtle up with Holy Shield, Consecration, Sanctuary, and Retribution Aura and let them kill themselves on me. Your only limitation is the one you always have when soloing: you may run out of mana. If you start hurting too badly, bubble and bandage. Your heals are horribly inefficient so don't waste mana on them unless you absolutely have to.